Met can stay in your system for a while, but the symptoms should stop soon after you stop taking the pills. If the insulin was working for you, I would definitely go back for it. Don't be surprised if, at least for a while, you would need a lot less. The insulin sensitizing properties of the met will last for a while but then will fade away as the drug gets out of your system.

On a side note, have you been tested for type 1? I don't know how long you were a prediabetic, but when you are an adult, you can appear to be type 2 for quite a number of years before requiring insulin.

As I said, if the insulin worked for you and you were happy enough on it, go back. You can get good control and be a bit more free in your diet.